The use of ultrasound to measure flow is well established with many installations worldwide in chemical plants, petrochemical plants, refineries, and so forth. A number of ultrasonic flowmeters have been developed, including transit-time based systems built as a one-piece “drop-in” flowcell with wetted transducers. In such systems, one or more transmitting transducers and one or more receiving transducers are aimed towards a medium flowing through the flowcell. An input voltage is applied to a transmitting transducer (transmitter) to cause it to transmit ultrasonic waves into the medium. These waves are received by a receiving transducer (receiver) and converted into an output voltage. The “time of flight” of the waves is determined by comparing the time at which the input voltage is applied to the time at which the output voltage is received.
The time required for an ultrasonic signal to travel against the flow (i.e., upstream), tup, is longer than that required to travel with the flow (i.e., downstream), tdn. The difference between upstream and downstream traveling times, Δt, is directly proportional to the flow velocity. The operation of an ultrasonic flowmeter strongly depends on the timing of tup, tdn, and Δt. The measurements of tup, tdn, and Δt conversely rely on the quality of the received ultrasonic signal, e.g., the signal-to-noise ratio (SNR).
In general, it is more challenging to apply ultrasonic techniques to gas flow measurement than liquid for a variety of reasons, including the much lower acoustic impedance, higher Mach numbers, higher turn-down ratios, and larger pressure variations associated with gas flow measurement. For example, the conversion of an electrical pulse to an ultrasonic signal in a gas medium at 0 psig, via a piezoelectric crystal in a transducer, is very inefficient. As a result, the acoustic signal transmitting through the gas is very small and needs amplification. The amplification amplifies both the acoustic signal through the gas and the unwanted noise escaping from the side and back of the transmitting transducer through a solid path (e.g., the wall of the flowcell) to the side and back of the receiving transducer. In other words, acoustic noise emitted from the transmitting transducer is coupled to the flowcell and, ultimately, to the receiving transducer. This noise (sometimes referred to as “short circuit noise”) generally does not carry any useful information about the fluid flow and thus contributes to the overall noise of the system and reduces the SNR. A high SNR is desired to make accurate and reliable flow measurements.
Accordingly, a need exists for transducer systems with reduced acoustic noise coupling.